This application claims the benefit of Japanese Patent Application Number 2012-245497 filed on Nov. 7, 2012, the entirety of which is incorporated by reference.
1. Technical Field
The present invention relates to main spindle devices of machine tools in which a plurality of bearings rotatably supporting a main spindle of the machine tool are placed inside a housing with a pressure receiving member interposed between the bearings and the housing. A pressure chamber, which a pressure medium pressing the pressure receiving member toward the bearings is supplied to, is provided in the housing.
2. Background Art
For example, Japanese Patent Application Publication No. H05-138408 (JP H05-138408 A) discloses a main spindle device of a machine tool in which clearance between a main spindle housing and each of outer races of bearings rotatably supporting a main spindle can be controlled by a simple configuration to prevent reduction in rigidity of the main spindle in a low speed rotation range. In the main spindle device of JP H05-138408 A, a pressure chamber is formed between the main spindle housing and an outer race spacer by a recessed portion of a thin ring-shaped member and the main spindle housing. The thin ring-shaped member is elastically deformed by pressure fluid supplied to the pressure chamber, and thus eliminates the clearance between the outer race of each bearing and the main spindle housing. Accordingly, reduction in rigidity of the main spindle in the low speed rotation range can be prevented.
Japanese Utility Model Application Publication No. H06-21803 (JP H06-21803 U) discloses a main spindle device of a machine tool which includes a preload adjustment device that applies preload to a plurality of bearings rotatably supporting a main spindle in a housing, in order to prevent reduction in rigidity of the main spindle. In the main spindle device of JP H06-21803 U, an annular pressing body that applies external force in the central axis direction of the main spindle is provided outward of outer races of the bearings. During low speed rotation of the main spindle, pressure oil is supplied to each pressurizing chamber formed in the annular pressing body so as to face the outer race of a corresponding one of the bearings. The annular pressing body deformed by the supply of the pressure oil presses the outer race of each bearing. This pressing force acts in the central axis direction of the main spindle and presses each bearing against the main spindle, whereby reduction in rigidity of the main spindle can be prevented.
However, in the main spindle device of JP H05-138408 A, the pressure chamber is formed between the main spindle housing and the outer race spacer, has a shape that is long in the axial direction of the main spindle, and a large capacity. This hinders uniform control of the pressure of the pressure fluid. In this case, the thin ring-shaped member subjected to the pressure of the pressure fluid cannot be uniformly elastically deformed. Accordingly, if the clearance is left in any region between the main spindle housing and the outer races of the bearings, the bearing cannot be sufficiently pressed against the main spindle in this region, and rigidity of the main spindle may not be increased.
In the main spindle device of JP H06-21803 U, the pressure oil is supplied to each pressurizing chamber through each branch oil passage that branches off from a single oil passage. The oil pressure of the pressure oil may vary between the pressurizing chamber located close to a pressure oil supply source and the pressurizing chamber located far away from the pressure oil supply source. In this case, the bearings are not uniformly pressed against the main spindle by the oil pressure. Thus, rigidity of the main spindle may not be increased in a region where the bearing is pressed by a low oil pressure.